In Long Term Evolution (LTE), the scheduler is placed in the eNodeB and the Medium Access Control (MAC) layer. The scheduler assigns radio resources, also called Resource Blocks (RB). The user equipments find out where to listen or where to send by listening for downlink (assignments) and uplink (grants) on the Physical Downlink Control Channel (PDCCH). Also, information concerning which transport format to use is comprised within the assignment and grant, respectively.
The radio downlink is the transmission path from a base station, e.g. an eNodeB to a terminal, or a User Equipment (UE) as the terminal also may be referred to as. The uplink is the inverse of a downlink, i.e. the transmission path from the terminal to the base station.
However, as the eNodeB schedules uplink transmissions while the buffers are located in the terminal, the terminal has to notify the eNodeB that it has data that it would like to transmit. The terminal supplies the eNodeB with information about the data in its buffers using two mechanisms; a 1-bit scheduling request (SR) or buffer status reports (BSR). Scheduling requests are transmitted on a control channel such as e.g. Physical Uplink Control Channel (PUCCH) or Random Access Channel (RACH). This process is illustrated in FIG. 1, which depicts prior art uplink scheduling. Buffer status reports are however transmitted on a data channel such as Physical Uplink Shared Channel (PUSCH) mostly together with user data.
If the terminal has a valid PUCCH resource for scheduling request configured in any transmission time interval (TTI) it sends a one bit scheduling request when the timing is right. Otherwise it initiates a random access procedure and cancels all pending scheduling requests.
The terminal is only allowed to use the PUCCH at predefined points in time determined by the Dedicated Scheduling Request (D-SR) interval. The delay between the actual generation time of a data packet such as e.g. a Voice over the Internet Protocol (VoIP) packet and the sending of the D-SR can thus become as large as the D-SR interval.
In the present context, the generation time is defined as the actual time when the data was put in the transmit buffer at the terminal and arrival time is defined as the time when the eNodeB receives the scheduling request.
When using service aware buffer estimation such as e.g. VoIP aware buffer estimation, the generation time is valuable.
In VoIP, the buffer estimation algorithm moves between two states, SID and TALK and a state change should preferably occur when the codec switches between the corresponding states. The TALK state is a proactive buffer estimation state which guesses when the next voice frame will arrive and which size it will have, while the SID state is a passive state that expects Scheduling Requests when data has arrived for a user.
As voice frames arrive every 20 ms to the terminal buffer using for instance Adaptive Multi Rate (AMR), the better the algorithm knows the generation time, the more exact it will predict the buffer size and the delay of the voice frame.
The larger the D-SR interval is, the larger the difference between the generation time of the VoIP packet and the arrival time noted by the eNodeB may be. This makes it harder to accurately predict the buffer state and schedule delay-sensitive services, increasing the need for explicit signalling and decreasing the efficiency of the uplink assignments.
The VoIP aware buffer estimator use the arrival of the D-SR, processing time is deducted, as the VoIP packet generation time, and this can be very different from the actual VoIP packet generation time. Furthermore, the eNodeB scheduler does not have correct packet delay information and may schedule the VoIP packet too late, especially in scenarios where the required VoIP delay is relatively short.
It is to be noted that the scheduling request is a scarce resource and thus the D-SR interval can be relatively long compared to the time between generation of voice packets at the terminal.